Robots that travel over terrain must be able to move under their own power to a location where their task is to be performed. Any mobile robot locomotion mechanism must support itself on the terrain and provide a propulsive force for motion. Possible locomotion candidates include mechanisms that roll, walk, or combine rolling and walking for so-called hybrid locomotion. (Track laying mechanisms are grouped here with rolling mechanisms because tracks are in continuous terrain contact and are analogous to large wheels.) The fundamental differences between rolling and walking mechanisms are the means by which support and propulsion are provided. Wheeled machines have rollers in continuous support contact with the terrain and propel themselves by generating traction forces parallel to the terrain surface. Alternately, walkers suspend themselves over the terrain on vertical contact points and maintain principally vertical contact forces throughout propulsion.
Early walking machines coordinated motion mechanically, placing feet or sets of feet in cyclic patterns. Because feet were placed blindly, irrespective of the underlying terrain, cyclic walking machines could not adapt to the terrain. On the other hand, terrain-adaptive walkers have the ability to individually place their feet at desired positions on the terrain. Individual foot placement and leg control allows a walker to optimize stability, follow a desired body trajectory and maximize forward progress even in very difficult terrains. Terrain-adaptive walkers are quite complex machines, typically having 4 or 6 legs and at least 12 degrees-of-freedom. In comparison, industrial robots usually have 4-6 degrees-of-freedom.
The Adaptive Suspension Vehicle (ASV) Waldron, K. J.; Vohnout V. J.; Pery, A.; McGhee, R. B. Configuration design of the adaptive suspension vehicle IJRR 1984, vol. 3, no. 2, 37-48, ODEX I Russell, M. ODEX I: the first functionoid. Robotics Age Sept./Oct. 1983, vol. 5, no. 5, pp. 12-18, and TITAN III Hirose, S.; Masui, T.; Kikuchi, H.; Fukuda, Y.; Umetani, Y. Titan III: a quadruped walking vehicle. 2nd Int. Symp. of Robotics Research Aug. 1984, Kyoto, Japan, pp. 247-253 are terrain-adaptive walkers. The legs of each of these walkers are based on pantograph mechanisms. The ASV and ODEX I incorporate legs with two-dimensional pantographs and a third rotary degree of freedom; the Titan III uses three-dimensional pantographs. A variety of "gaits"--leg coordination and control schemes--are used to advance the walkers over terrain. A common gait, the wave gait, advances sets of legs in a wavefront in the direction of travel. Existing terrain-adaptive walkers raise valid concerns regarding the difficulty of planning, energy efficiency and reliability.
Regardless of whether the walking machine is human or computer controlled, the planning required for selecting footholds for many feet can be quite cumbersome. The task of foothold selection is complicated by rugged and difficult terrain conditions. One method that reduces foothold planning is to use precomputed plans that automatically place feet to geometrically satisfy a gait with little or no consideration for the specifics of the underlying terrain. An alternate method to reduce planning uses a follow-the-leader gait in which the computer (or operator) only selects front (leading) feet placements. Trailing feet step next to or in the vacated footprints of leading feet.
The mechanism described herein uses unique overlapping gaits that at least halve the number of footfalls compared to a conventional walker (e.g., ASV) with similarly proportioned legs. In an overlapping gait, a recovering leg can fully or partially pass by a supporting leg without mechanical interference. The result is a large increase in stroke (distance from footfall to foot lift--relative to the body) over conventional walkers and a corresponding significant reduction in the number of footfalls. The wave and follow-the-leader gaits of conventional walkers can require that recovering feet are placed next to leading feet, thus requiring a terrain area large enough to accommodate adjacent footholds for both feet. Since overlapping gaits allow feet to pass by other feet, there is no need to establish a second foothold next to the first thus relaxing the difficulty of walk planning in terrains in which many areas are deemed unacceptable for foot placement. The outcome is a walking mechanism that can succeed in terrains that would frustrate or impede walkers that require more footholds and have the constraint that pairs of footholds be adjacent.
Improving overall energy efficiency is another motivation to reduce footfalls. Footfalls into most natural materials expend irrecoverable energy in the form of terrain deformation, and depending on the type of terrain (e.g., dry sand), a significant amount of energy may be lost. By reducing footfalls, overlapping gaits cut the energy that a walking machine dissipates to the terrain.
Most existing walkers are immobilized or severely impeded after the failure of one or more legs. A unique characteristic of the preferred embodiment of the walking mechanism described herein is that any foot can be placed at any point around the full periphery of the walker body. Therefore, any functional leg can reposition itself to substitute for any failed leg. Though ensuing gaits may have to be modified to account for the reduced number of operational legs, all zones around the body could be covered by operational legs.